The present invention relates to an aqueous pigment-dispersed paste for dispersing beforehand a pigment in an aqueous medium when water paint etc. is produced, and a process for producing it. The invention also relates to a process for producing a water paint composition in which the paste is used.
In producing a water paint, there has been commonly employed an aqueous pigment-dispersed paste in which a pigment has been dispersed in an aqueous medium. This is because, if the whole paint components containing a pigment is dispersed at one time, a dispersing process which usually consumes a lot of energy becomes long and efficiency in production greatly lowers. Because of this, using a dispersing agent, a pigment is beforehand dispersed in an aqueous medium at high concentration with stabilization.
Representative of the dispersing agent is a paste of this type are,
1 surface-active agents having a low molecular weight and an anionic, cationic, or nonionic character, and PA1 2 polymers (resin) having a medium or high molecular weight.
In the polymer having a medium or high molecular weight are generally known anionic polymers having an acidic functional group such as a (meth)acrylic acid monomer (see, for example, Japanese Official Patent Provisional Publication, showa 50-154328), and cationic polymers having a basic functional group such as a quaternary sulfonium group (for example, refer to Japanese Official Patent Provisional Publication, showa 63-23919). Also, Japanese Official Patent Provisional Publications, showa 59-227940, 60-81252, 60-81253, 60-86171, 60-92360, 60-123563, 61-235466, 63-154776, and 63-35613, disclose that polymers having both of an acidic and a basic functional groups can be used.
The forementioned surface-active agents do not have a sufficient dispersing stability of pigment because of the low molecular weight and are defective on paint film performance such as water-resistance and corrosion-resistance, because the forementioned surface-active agent remains in a coating film.
The paint film performance is improved, using a polymer of a medium or high molecular weight described in the above item 2, and by combining a dispersing agent into a binder component in a coating film. However, the forementioned polymer is generally sulbilized in a water by neutralizing it, in the case of an anionic polymer, with a basic compound in an amount of more than the equivalent and, in the case of a cationic polymer, with an acidic compound in an amount of more than the equivalent. As a result, each aqueous resin solution shows a basic property in the case of the anionic resin and an acidic property in the case of the cationic resin.
As is widely known, pigment particles in various types of aqueous solutions are positively charged when a pH of the aqueous solution is in a region more acidic than an isoelectric point of the pigment and are negatively charged when it is in a region more basic. This is because the isoelectric points of most pigments used for paints are in a neutral region, and an aqueous anionic polymer solution shows a basic character as mentioned above. Thus, the pigment particles are negatively charged and generate an electrostatic repulsive force with a negative electric charge of a resin. As a result, the dispersing stabilization due to adsorption of the resin is insufficient and the dispersibility and dispersion stability of the pigments are unsatisfactory.
Similarly, the pigment is positively charged in a cationic polymer solution and generates an electrostatic repulsive force with a positive electric charge of the resin. As a result, the dispersing stabilization due to adsorption of the resin is insufficient and the dispersibility and dispersion stability are unsatisfactory.
Furthermore, in Japanese Official Patent Provisional Publications, showa 59-227940, 60-81252, 60-81253, 60-86171, 60-92360, 60-123563, 61-235466, 63-154776, and 63-35613, attempts were made to improve the above defects by introducing a basic functional group into an anionic resin. In all these prior arts the same ideas as mentioned below were used, that is,
1) an appropriate balance between hydrophilic and lipophilic domains in resin is most important;
2) the hydrophilicity of the hydrophilic domain is controlled by copolymerizing an acidic and an acrylic monomer;
3) since the resin posses both an acidic and a basic functional groups, a wide range of pigment species are capable of finely dispersing.
However, in these arts a basic compound which is used for neutralizing an acidic functional group involved in the anionic resin is substantially prior in dissociation to a basic functional group involved in a resin and, therefore, an electrostatic attractive force between the resin and pigment is not revealed and the dispersing stability due to resin adsorption is insufficient and the dispersibility and dispersion stability of the pigment are not good.